In recent years, gallium nitride-based compound semiconductors have become of interest as materials for producing a light-emitting device which emits light of short wavelength. Generally, a gallium nitride-based compound semiconductor is grown on a substrate made of an oxide crystal such as a sapphire single crystal, a silicon carbide single crystal, or a Group III-V compound single crystal, through a method such as metal-organic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE).
At present, the crystal growth method that is most widely employed in the industry includes growing a semiconductor crystal on a substrate such as sapphire, SiC, GaN, or AlN, through metal-organic chemical vapor deposition (MOCVD). Specifically, an n-type layer, a light-emitting layer, and a p-type layer are grown on the aforementioned substrate placed in a reactor tube, by use of a Group III organometallic compound and a Group V source gas at about 700° C. to about 1,200° C.
After growth of the above layers, a negative electrode is formed on the substrate or the n-type layer, and a positive electrode is formed on the p-type layer, whereby a light-emitting device is fabricated.
Conventionally, such a light-emitting layer is formed from InGaN whose composition is controlled so as to modulate the light emission wavelength. The active layer is sandwiched by layers having a bandgap higher than that of InGaN, thereby forming a double-hetero structure, or is incorporated into a multiple quantum well structure on the basis of the quantum well effect.
In a gallium nitride-based compound semiconductor light-emitting device having a light-emitting layer of a multiple quantum well structure, when the thickness of a well layer is adjusted to 2 to 3 nm, satisfactory output is attained, but a problematically high operating voltage is required. In contrast, when the thickness of the well layer is 2 nm or less, the operating voltage is lowered, but the output is poor.
Further, a quantum dot structure as described below, in which light-emitting layers are formed in a dot form, is suggested to improve the emission output.
For example, in Japanese Laid-Open Patent Application (kokai) No. 10-79501, Japanese Laid-Open Patent Application (kokai) No. 11-354839, etc., a light-emitting device containing a light-emitting layer having a quantum dot structure is disclosed. The quantum dot structure is formed according to the anti-surfactant effect. According to Japanese Laid-Open Patent Application (kokai) No. 11-354839, the size of each light-emitting device is preferably such that 0.5 nm≦height≦50 nm, 0.5 nm≦width≦200 nm, and 106≦density≦1013 cm−2. In examples, the light-emitting device having a height of 6 nm and a width of 40 nm is produced.
However, a part which is not covered with quantum dots is a region having an extremely low resistance in comparison with a dot region, whereby the current is preferentially applied thereto and the non-dot part does not contribute to light-emission. Therefore, the quantum dot structure thus suggested has a problem that the emission output with respect to the applied current is decreased as a whole, although the emission efficiency of each light-emitting dot is improved.
Japanese Laid-Open Patent Application (kokai) No. 2001-68733 discloses a quantum box structure containing In. According to Japanese Laid-Open Patent Application (kokai) No. 2001-68733, the quantum box structure is formed by annealing, in hydrogen, a quantum well structure once produced, whereby the well layer is sublimated. In examples, the quantum box has a size of not more than 200 Å, for example, the size of 20 Å×20 Å×20 Å. The density of the light-emitting box is not defined. However, it can be seen from the drawings that the area which is covered with the light-emitting box is the same as or smaller than the area which is not covered with the light-emitting box.
In short, in these technologies, it is defined that the region where no quantum dot or quantum box is formed has a structure where no dot or box is formed. Therefore, the region where no quantum dot or quantum box is formed is a region having an extremely low resistance in comparison with the region where a quantum dot or a quantum box is formed, whereby the current is preferentially applied to the region where no quantum dot or quantum box is formed.
The structure, in which no light-emitting elements are provided in a region not covered with a dot or a box, has a problem that the emission output is decreased although the operating voltage is effectively decreased, and it cannot be practically used.
Japanese Laid-Open Patent Application (kokai) No. 2001-68733 also discloses that a quantum box structure is fabricated by forming a conventional quantum well structure and annealing the structure in hydrogen, thereby decomposing an InGaN crystal provided on through-hole dislocations. However, annealing a quantum well structure in hydrogen induces release of In from a portion intended to serve as a quantum box structure, thereby blue-shifting the emission wavelength, which is not preferred.
Also, in US Patent Application Publication No. US2003/0160229A1, a light-emitting layer having a multiple quantum well structure in which the thickness of the well layer is partially changed is suggested to obtain a highly efficient light-emission. In the specification thereof, at least from the attached drawings, it is found that all the well layers are irregular in thickness.